[22.05] Link between Mass-loss and Variability Type for AGB Stars?

Z. Ivezi\'{c}, G. R. Knapp (Princeton University)

We find that AGB stars separate in the 25-12 vs. 12-K
color-color diagram according to their chemistry (O, S vs.
C) and variability type (Miras vs. SRb/Lb). While
discrimination according to the chemical composition is not
surprising, the separation of Miras from SRb/Lb variables is
unexpected.

We show that ``standard'' steady-state radiatively driven
models provide excellent fits to the color distribution of
Miras of all chemical types. However, these models are
incapable of explaining the dust emission from O-rich SRb/Lb
stars. The models can be altered to fit the data by
postulating different optical properties for silicate
grains, or by assuming that the dust temperature at the
inner envelope radius is significantly lower (300-400 K)
than typical condensation temperatures (800-1000 K), a
possibility which is also supported by the detailed
characteristics of LRS data. While such lower temperatures
are required only for O- and S-rich SRb/Lb stars, they are
also consistent with the colors of C-rich SRb/Lb stars.

The absence of hot dust for SRb/Lb stars can be interpreted
as a recent (order of 100 yr) decrease in the mass-loss
rate. The distribution of O-rich SRb/Lb stars in the 25-12
vs. K-12 color-color diagram shows that the mass-loss rate
probably resumes again, on similar time scales. It cannot be
ruled out that the mass-loss rate is changing periodically
on such time scales, implying that the stars might oscillate
between the Mira and SRb/Lb phases during their AGB
evolution as proposed by Kerschbaum et al. (1996). Such a
possibility appears to be supported by recent HST images of
the Egg Nebula obtained by Sahai et al. (1997), the
discovery of multiple CO winds reported by Knapp et al.
(1998), and long-term visual light-curve changes detected
for some stars by Mattei (1998).

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